1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic method, such as a printer, a copier or a facsimile.
2. Related Background Art
Conventionally, an image forming apparatus of an electrophotographic method exposes a uniformly charged electrophotographic photosensitive member (hereafter referred to as a “photosensitive member”) according to an image information signal and forms an electrostatic image (latent image), which is developed with a developer into a toner image to be eventually transferred to a recording material such as paper. Thereafter, the toner image transferred on the paper is fixed by using heat and pressure. The photosensitive member is cleaned by removing the developer and so on left on the transfer with cleaning means so as to move on to a charging step again and form the image.
Such an image forming apparatus may have charging means for uniformly charging the photosensitive member by using a discharge phenomenon such as a corona discharge or a discharge between minute gaps near a contact portion of a roller and the photosensitive member (discharge means). The discharge means as above may also be used as transfer means for transferring the toner image formed on an image bearing member (such as a photosensitive member or an intermediate transferring medium) to a transfer material such as the recording material or intermediate transferring medium. A discharge by using such discharge means generates discharges such as nitrogen oxide (hereafter referred to as “NOx”) and ozone, which partially adhere to a surface of the photosensitive member.
Thus, of the discharges adherent to a surface layer of the photosensitive member, NOx remaining on the surface layer of the photosensitive member generates nitric acid by reacting with moisture in the air or generates metal nitrate by reacting with a metal. If the nitric acid or nitrate thus generated is formed as a thin film on the surface of the photosensitive member, an electrical resistance value on the surface of the photosensitive member is reduced by moisture absorption of the nitric acid or nitrate. There are the cases where the electrostatic image formed on the photosensitive member is thereby destroyed and a quality of a formed image is lowered. Under a high-humidity environment in particular, an abnormal image as if the image is deleted (image deletion) is apt to be generated.
In the case of using a conventional organic photosensitive member, the surface layer of the photosensitive member is scraped away by an infinitesimal amount, on using a two-component developer including nonmagnetic toner particles (toner) and magnetic carrier particles (carrier) as the developer, by sliding the photosensitive member with a magnetic brush of a magnetic carrier in a development portion (development nip) or sliding the photosensitive member with a cleaning member such as a blade-like member for removing the toner left on the transfer remaining on the photosensitive member. And the above-described discharges and the nitric acid or metal nitrate generated by reaction thereof are removed on having the surface layer of the photosensitive member scraped away. Thus, it is conventionally possible to suppress generation of the abnormal image by Nox to a certain extent.
However, there is a problem of reducing life of the photosensitive member in the case of thus scraping away the surface of the photosensitive member with the magnetic brush of the magnetic carrier in the development portion or the cleaning member.
Thus, there has been a devised method whereby the photosensitive member having an photoconductive layer on its surface has a layer thickness thereof thickened to earn the life of the photosensitive member to a certain extent even in the case where sliding is performed with the magnetic brush of the magnetic carrier in the development portion or the cleaning member. If the layer thickness of the photoconductive layer is excessively thickened by this method, however, there is a problem that diffusion of optical carrier occurring on image exposure is increased and resolution is reduced. Therefore, it is difficult, in this case, to extend the life of the photosensitive member while maintaining a higher image quality. Furthermore, if a sliding force of the magnetic brush of the magnetic carrier in the development portion or the cleaning member is increased, there is a possibility of generating a scratch affecting image forming on the surface layer of the photosensitive member. If the sliding force of the cleaning member is increased, there is also a possibility that the cleaning member itself may have a defect such as a chip leading to insufficient cleaning.
To attain both the above-mentioned higher image quality and extended life, there is a proposal of a photosensitive member having a harder surface which can reduce a scraped-away amount of the photoconductive layer itself of the photosensitive member even in the case where sliding is performed with the magnetic brush of the magnetic carrier in the development portion or the cleaning member to remove the discharges (Nox). As for such photosensitive members, there are a photosensitive member having a protective layer provided on the surface layer to protect an organic photoconductive layer and an α-Si photosensitive member. These photosensitive members have their surface layers hardened and so the scraped-away amount due to mechanical sliding is naturally reduced. Therefore, it is possible to reduce a film thickness of the photoconductive layer and protective layer on creation and decrease the diffusion of optical carrier occurring on image exposure so as to attain both the higher image quality and extended life.
However, in the case of reducing the scraped-away amount of the photoconductive layer itself, there is a possibility, according to the conventional method, that it may become difficult to scrape away the film of the nitric acid or nitrate formed on the photosensitive member. It is because, according to the conventional method, the surface layer of the photosensitive member in a lower part of the film of the nitric acid or nitrate is scraped away even though a little so as to scrape away and remove the film of the nitric acid or nitrate formed on the photosensitive member. To be more specific, in the case where the scraped-away amount of the surface layer of the photosensitive member is relatively large, it is possible to scrape away the film of the nitric acid or nitrate in its entirety including the surface layer of the photosensitive member. However, it becomes difficult to scrape away the film of the nitric acid or nitrate if the scraped-away amount of the surface layer of the photosensitive member is reduced.
Thus, there are proposals of various methods of solving the problems of the discharges by working on the discharges before they adhere to the photosensitive member rather than the above-mentioned methods of scraping away the surface of the photosensitive member. For instance, Japanese Patent Application Laid-Open No. H10-340030 discloses an image forming apparatus for preventing reduction in charging characteristics due to ozone by using a method of exhausting the ozone generated by the discharge outside the apparatus by exhaust means. Japanese Patent Application Laid-Open No. H05-303244 discloses an image forming apparatus for preventing the NOx generated by the discharge from becoming the nitric acid by using a method of providing heating means for preventing dew condensation which prevents dew drops from being generated on the photosensitive member.
In addition, there are proposals of a charging device for decomposing the NOx generated by the discharge by concurrently providing a creeping glow discharge device on the same board of a discharge electrode for charging, a corona generating device for absorbing the NOx by coating a shield as a component of the corona generating device with an alkaline film for neutralizing the NOx or a corona generating device provided with a photocatalytic substance capable of decomposing the discharges such as the ozone and NOx in a casing of the corona generating device in a form of a porous body structure.
As for the above methods, however, the devices themselves require space and cost. Therefore, the image forming apparatus in demand is the one of a simple configuration capable of removing the discharges adherent to the photosensitive member while extending the life of the photosensitive member.